David Cameron and Barack Obama could tighten sanctions against Syria to increase the pressure on president Bashar Assad, Downing Street has said.

A No 10 spokesman said the Prime Minister and the US president were "disappointed" at the failure earlier this month to agree a United Nations Security Council resolution (UNSCR) condemning the regime's brutal crackdown on anti-government protesters who took to the streets 10 months ago as popular uprisings swept the Arab world.

The pair have previously called on Assad to step down, and they talked about the latest violence in a transatlantic phone call on Monday night.

A Downing Street spokesman said: "The leaders expressed disappointment at the Russian and Chinese vetoes of the draft UNSCR on February 4.

"They agreed on the need for international unity against the regime's attacks on its own citizens, including further action at the UN and a broad and strong coalition in the new Friends of Syria group.

"They discussed the possibility of increasing the pressure on the Assad regime through additional sanctions."

No 10 said Mr Obama also praised Mr Cameron's stance on Somalia ahead of next week's London conference on how best to tackle "piracy, terrorism, conflict, poverty and famine" in the war-torn nation.